As metal-oxide-semiconductor transistors (MOSFETs) are aggressively scaled to smaller size, the performance of such MOSFETs may be significantly limited by short channel effects and gate leakage current. Short channel effects arise if channel lengths of MOSFETs are reduced by scaling in an attempt to increase both operational speed and a number of MOSFETs per chip. Threshold voltages of MOSFETs become more difficult to control, due at least in part to a modification of the threshold voltage caused by the shortening of the channel lengths as a result of scaling. With regards to gate leakage current, scaling reduces a thickness of a gate oxide of a MOSFET, but the decreased thickness of the gate oxide causes an amount of the gate leakage current to increase during an OFF-state of the MOSFET. The increased amount of gate leakage current disadvantageously results in increased power consumption.
In addition to short channel effects and increased gate leakage current, there are other challenges with MOSFETs. As one example, MOSFETs have a high subthreshold swing, typically greater than 60 mV/decade. The subthreshold swing is generally defined as a level of gate voltage to change a drain current by one order of magnitude (e.g., by one decade), and with scaling to reduce a MOSFET's size, the subthreshold swing increases. A disadvantageous consequence of an increased subthreshold swing is that a higher power supply voltage may be needed to turn ON the MOSFET. Another disadvantage of an increased subthreshold swing is an increase in leakage current during an OFF-state of the MOSFET. Supply voltage scaling is another example of a challenge with MOSFETs. It is often difficult to scale (decrease or increase) a level of supply voltage (e.g., VDD) provided to a MOSFET based on the particular application or use of the MOSFET. Thus, VDD scaling limitations may reduce the capability to provide an optimum supply voltage VDD to a reduced-size MOSFET for a low-power digital application.
In comparison to MOSFETs, tunneling field-effect transistors (TFETs) having a gate-modulated Zener tunnel region may provide subthreshold swings of less than 60 mV/decade and may operate at a lower supply voltage VDD. Thus, TFETs are considered as potential candidates to replace MOSFETs in low-power digital applications.
However, most silicon (Si)-based or silicon-germanium (SiGe)-based TFETs exhibit low ON-state current. For example, there is a high tunneling barrier in the tunnel region of Si-based and SiGe-based TFETs, due at least in part to the large bandgap of the material of the tunnel region. This high tunneling barrier is characterized by a smaller amount of electrons moving through the tunnel region, thereby resulting in reduced ON-state current that in turn results in slower operating speed of the Si-based and SiGe-based TFETs.